1. Field of the Invention
This invention is concerned with improving the coupling response of multi-component seismic signals derived from sensors implanted on the water bottom. In general, the invention is concerned with the spectral balancing of the multi-component signals in a component-consistent manner with respect to the geometry of the sources and receivers.
2. Discussion of Relevant Art
Seismic exploration studies often involve use of both P-waves and S-waves. In marine operations, although compressional waves propagate through the water as pressure fields, shear waves do not because water has no shear strength. Therefore shear-wave studies, such as used in vertical rock-fracture studies, conducted in a water environment require use of 3-component motion-sensitive sensors, such as geophones in addition to pressure-sensitive hydrophones which normally are used in marine exploration. The geophones are planted directly on the water bottom using ocean bottom cables (OBC). For reasons to be explained later, a hydrophone is included with the OBC instrument package.
Please refer to FIG. 1 where a plurality of seismic sensor packages 10.sub.0, 10.sub.1, 10.sub.2, . . . , 10.sub.i (j=3,4,5, . . . , n, where n is an integer) are shown laid on the bottom 12 of a body of water, 14. The sensor packages are spaced-apart by a desired separation such as 10 meters. The sensor packages are preferably multicomponent. A 4-component sensor generates electrical signals that are proportional to particle velocity of the water bottom material in each of three spatial axes and to differences in water pressure. The particle-velocity sensors are responsive to seismic waves in general and in particular to both compressional and shear waves. Hydrophones are sensitive only to pressure wavefields.
The sensors are mechanically and electrically coupled to a sectionalized ocean-bottom cable (OBC) 16 of any well-known type, which may be many kilometers long. The OBC includes communication channels, which may be electrical, optical or ethereal, for transmitting the individual sensor signals to suitable instrumentation of any well-known variety mounted in a service vehicle of any appropriate type. One or both ends of a cable may be marked by a buoy, such as 18, at the water surface 11, for later recovery. For three-dimensional (3-D) areal surveys, many cables may be laid out side-by side in parallel, perhaps 25 meters apart, in a wide swath.
Usually, the cables and sensor packages (hereinafter referred to as receivers) are laid out over the area to be surveyed by a cable-tender boat. At some later time, a service ship such as 20, recovers one or more cables, such as 16 from the water bottom. The signal communication channels are connected to multi-channel recording instrumentation of any desired type, generally shown as 21, installed in the ship 20, for receiving and partially processing seismic signals.
An acoustic sound source 26 is fired at each of a plurality of designated source locations distributed over an area of interest. The act of firing an acoustic source is defined as a shot. The source locations are preferably spaced apart by an integral multiple of the receiver spacings. Source 26 radiates wavefields such as generally shown by 28 and 30 to insonify subsurface earth layers such as 32, whence the wavefield is reflected back towards the surface as reflected wavefield 34. The receivers 10.sub.j intercept the mechanical earth motions, convert those motions to electrical signals and send those signals through the communication channels, one channel per receiver or receiver group, to multi-channel recording equipment 21 of any desired type, in ship 20.
A wavefield may propagate through a volume of the earth along trajectories such as a refracted path (not shown), a direct travel path such as 36 or along reflected-ray travel paths such as 38, 38' and 38" to the respective receivers 10.sub.j. The different wavefield trajectories intersect at the source location. The recorded data are often presented in the form of time-scale traces, by way of example but not by way of limitation, one trace per communication channel.
A collection of time-scale traces resulting from a single source activation (shot 26) that insonifies a plurality of receivers 10.sub.1 -10.sub.j such as in FIG. 1, constitutes a common shot gather. On the other hand, with reference to FIG. 5, a collection of time-scale traces as recorded by a single receiver 10.sub.j after illumination by a plurality of spaced-apart shots 26.sub.i, 26.sub.i+1, 26.sub.i+2 constitutes a common receiver gather. Here, the different wavefield trajectories intersect at the common receiver. Other gathers such as common offset and common mid-point are known.
Since a shot and a receiver can always be interchanged by reason of the principle of reciprocity, given common processing and instrumentation, the two types of gather are equivalent. Since it is immaterial whether we are dealing with a common receiver gather or a common shot gather, throughout this disclosure, for simplicity, we will refer to gathers in general by the generic term: common trace gathers. By definition, a common-trace gather is a collection of time-scale recordings of signals representative of the traveltimes of a plurality of wavefields that have propagated through a volume of the earth over different trajectories which have a common intersection.
The horizontal separation between a source and a receiver, is defined as the offset, h. Typically in 3-D operations ship 20 occupies a convenient central location, interconnected with a plurality of cables and receivers, while a second shooting ship (not shown) actually visits the respective designated survey stations to generate common trace gathers. Typically, for OBC surveys, the data are digitally sampled at time intervals such as every four milliseconds, over a frequency passband of 5 Hertz to Nyquist. Other sample rates and passbands of course may be used.
FIG. 2 is a close-up, X-ray-like side view of a 4-component receiver 10.sub.j. The particle-velocity receivers (geophones) are polarized in-line (x axis), unit 42, cross-line (y axis), unit 44, vertically (z axis) unit 40. The double-headed arrows indicate the axis of maximum sensitivity. The pressure receiver (hydrophone), 43, responds to pressure differences as shown by the imploding arrows. Preferably, the two horizontally-polarized receivers respond to S-waves while the vertically-polarized receiver and the spatially unpolarized hydrophone respond to P-waves.
A 4-component receiver is customarily packaged in a single elongated case. The particle-velocity sensors are gimbal-mounted so as to become automatically aligned along their mutually orthogonal axes after deposition on the sea floor. For good and sufficient reasons, the case containing the receiver components is usually cylindrical. Cable 16 is relatively heavy. Secured to the fore and aft ends of the elongated receiver case, the cable 16 firmly holds a typical multi-axis receiver unit, 10.sub.j, to the sea floor 12. The in-line receiver component 42 is well coupled to sea floor 12 because of the inherent stability of the elongated case along the in-line direction. That happy situation in not true, however, for the cross-line receiver component 44.
Please refer to FIG. 3 which is an X-ray-like cross section of multi-component receiver 10.sub.j taken along line 3-3', looking back towards ship 20. Because of its cylindrical shape, case 10.sub.j can roll infinitesimally from side-to-side as shown by curved arrows 46, but water currents and other disturbances can cause the receiver case to roll and shift laterally in the cross-line direction as shown by arrows 48, 48'. Those disturbances do not affect the in-line receiver signal component because of its polarization direction but they can introduce severe noise to the cross-axis and to the vertically-polarized signal components.
Since the hydrophone is unpolarized, coupling noise is not a problem but as before stated, a hydrophone is transparent to S-waves.
FIG. 4 is the 4-component receiver 10.sub.j as viewed from above along line 4-4' of FIG. 2.
A method for correcting poor coupling of a logging sonde in a borehole was described in a paper by J. E. Gaiser et al., entitled Vertical Seismic Profile Sonde Coupling, published in Geophysics n. 53, pp 206-214, 1988. Although that method is not directly applicable to 3-D seismic exploration, it is of interest because it demonstrates the evil effects of poor coupling of a sensor to the ground.
In a paper entitled "the Recovery of True Particle Motion from Three-component Ocean Bottom Seismometer Data by J. D. Garmany, published in the Journal of Geophysical Research, v. 89,n. B11, p.9245-9252, 1984, a theory and method is presented to correct for poorly coupled ocean bottom seismometers (OBS) that considers the OBS as a Hamiltonian system having many degrees of freedom with only three inputs and three outputs. A matrix Levinson recursion method is used to solve for z-transform operators applied to the recorded signals to recover true particle motion. Although this method makes no assumptions about a complicated system of springs and dashpots, it also does not incorporate a specific geometry of the source or receiver system on the water bottom.
G. F. Moore et al, in Seismic Velocities at Site 891 from a Vertical Seismic Profile Experiment, published in Proceedings of the Ocean Drilling Program, Scientific Results, v.146, pt.1, p 337-348, offered another method wherein they compute vector deconvolution operators in the time domain for borehole seismic data. They use assumed linear polarization directions of the windowed direct arrival (source-receiver directions) to solve for operators in the least squares sense. However it does not consider the specific receiver geometry for limiting solutions to particular directions of motion.
In an article entitled An Improved Method for Deriving Water-bottom Reflectivities for Processing Dual Sensor Ocean Bottom Cable Data, Expanded Abstracts paper SA3.2, pages 987-990, 1995, 65th Annual Meeting of the Society of Exploration Geophysicists, J. Paffenholz et al, explain that dual-sensor processing in which velocity and pressure signals are recorded in OBC surveys requires the use of scale factors that are functions of the ocean bottom reflectivity. They describe a new method to derive the ocean bottom reflectivity from production seismic data. The method eliminates the need for a separate calibration survey and is stable in the presence of random and shot-generated noise. This method may be adapted for use in processing 4-component seismic receiver data.
U.S. Pat. No. 5,524,100 issued Jun. 4, 1996 to J. Paffenholz and which is incorporated herein by reference teaches that pressure and velocity signals may be combined. The combined signal is transformed into the frequency domain and multiplied by the inverse Backus operator or the combined signal is convolved with the inverse Backus operator and an optimization algorithm is utilized to solve for water bottom reflectivity. Pressure and velocity seismic signals are combined and the combined signal is multiplied by the inverse Backus operator containing the water bottom reflection to eliminate first order peg-leg multiples.
There is a long-felt need for a method for measuring and suppressing signal distortion attributable to poor water-bottom coupling of a component of an ocean-bottom, cable-mounted, multi-component seismic receiver and for balancing the spectral response of the respective components.